1. Technical Field of the Invention
This invention relates to hydrostatic power steering systems for road and industrial vehicles.
2. Prior Art
Said systems comprise a hydraulic actuator arranged to operate in two opposing directions, generally in the form of a cylinder-piston unit with a through rod, which is connected to the steering frame. The hydraulic actuator is fed via a cylindrical distributor which directs controlled quantities of pressurized hydraulic fluid supplied by a pump to the actuator, and directs the fluid leaving the actuator to discharge.
The quantity of fluid fed to the actuator is controlled by a dispenser of orbital type, the rotor of which is connected to the steering wheel. The distributor assumes three operative positions, in one of which, the so-called neutral position, the pressurized fluid originating from the pump is fed directly to discharge, whereas in each of the other two the pressurized fluid is fed to the hydraulic actuator which controls the steering in one or the opposite direction, the fluid leaving the hydraulic actuator being fed to discharge.
The distributor is generally of the cylindrical barrel type, comprising two sealedly coupled cylindrical bodies able to assume different relative angular positions.
The inner body (the spindle) is axially hollow and is coupled to the vehicle steering wheel.
The outer body, commonly known as the barrel, is torsionally connected to the spindle by elastic means, limited to initial small rotations, and by rigid means beyond small rotations.
Both the bodies are provided with holes and axial grooves distributed on their surface, which assume mutual correspondence on the basis of the relative angular position between the spindle and barrel.
The elastic means maintain the barrel and spindle in that relative position corresponding to neutral, whereas operating the steering wheel modifies the relative mutual position between the barrel and spindle to direct the hydraulic fluid along one or other direction corresponding to the two different steering directions.
The connection between the steering wheel shaft and the spindle is rigid, whereas that between the spindle and barrel is elastic with the result that operating the steering wheel causes a phase displacement between the spindle and barrel which is maintained constant while the steering wheel is rotating, whereas when steering wheel rotation ceases the elastic means return the barrel and spindle into phase.
The hydraulic fluid fed to the hydraulic actuator is measured by an orbital dispenser, the rotations of which correspond to the rotations of the spindle and hence to the rotations of the steering wheel. In this manner, while the steering wheel is rotated in one or other direction, the fluid continues to be fed in controlled quantities to the hydraulic actuator which operates the steering, and the wheels continue to change direction.
When however the steering wheel is maintained stationary, controlled feed to the hydraulic actuator ceases and the wheels maintain their steered position.
Known devices of the aforesaid type present a drawback consisting of the fact that the steering wheel has to continue to be rotated in order to continue to steer the vehicle wheels; in this respect, if rotation of the steering wheel is halted, the barrel and spindle return in phase, and the wheels remain locked in that steering angle which they have attained.
This method of operation is extremely uncomfortable during those vehicle operations involving very tight steering movements to be made one following the other in one and the other direction.
To effect said movements the driver is obliged to continue to rotate the steering wheel for several turns in one direction and then to immediately rotate it in the opposite direction for the same number of turns.
A requirement therefore exists for a hydrostatic power steering device which, besides providing for the normal operation of the state of the art, also enables complete steering movements to be effected by modest rotations of the steering wheel.